A method for measuring the temperature of a fluid in a measuring tube is known from DE 44 42078 A1. According to this method, a first ultrasonic signal is emitted into a measuring tube in the direction of flow of the fluid and received, run-time Tdown of the first ultrasonic signal being measured. Furthermore, a second ultrasonic signal is emitted into the measuring tube against the direction of flow of the fluid and received, run-time Tup of the second ultrasonic signal being measured. Flow velocity v of the fluid in the measuring tube is ascertained from both run-times Tdown and Tup. Flow velocity v of the fluid is determined from speed of sound C0, and temperature T of the fluid flowing in the measuring tube is concluded from speed of sound C.
A mass flow rate meter is discussed in DE 42 37 907 A1. An ultrasonic flow rate meter for measuring the mass flow rate includes at least two ultrasonic transducers, which are attached to a wall or in the walls of a flow channel. The two ultrasonic transducers are connected to an analysis unit for measuring the volume flow rate. A thermal sensor is provided on or in the walls of the flow channel, which is used for measuring the temperature. For analyzing the measurement results, the meter is connected to the same or a further analysis unit, this analysis unit determining the density of the material in the flow channel at least in principle. The analysis units are implemented in such a way that the thermal sensor allows at least approximate measurement of the mass flow rate as an auxiliary if the ultrasonic transducer fails.
The thermal sensor may be implemented as a group of hot wires, as a hot film, as an NTC resistor, as a PTC resistor, or also as a micromechanical thermal sensor.
The DE 42 37 907 A1 document has the disadvantage that, if the ultrasonic transducer fails, only an at least approximate measurement of the mass flow rate is performed via the thermal sensor, which results in imprecise results for exactly determining the charge of combustion air in the combustion chambers of an internal combustion engine. To allow optimum combustion, extremely precise information about the air quantity contained in the combustion chambers of the internal combustion engine is necessary to inject a fuel quantity tailored thereto, which is ideally in stoichiometric ratio to the air quantity present in the combustion chamber.
Furthermore, ultrasonic measuring units are known from the related art, in which two diametrically opposed ultrasonic transducers are provided in a measuring tube in which a gaseous medium flows. These alternately or simultaneously transmit ultrasonic signals. The resulting run-times of ultrasonic signals t1 and t2 are measured in the flow direction of the gaseous fluid and opposite to the flow direction of the gaseous fluid to measure the flow rate of the flowing medium.